Spraying a coating material onto the surface of a body is commonly done. For example, interior surfaces of metal beverage cans are coated to preserve the flavor of the contents from being changed due to contact with a metal surface. A variety of spray systems have been developed over the years. In the can industry, can interiors are sprayed using one or more spray applicator devices or spray guns having one or more nozzles positioned near the can interior. Material is sprayed onto the can surfaces typically while the can is rotated. Can surfaces may include interior and exterior surfaces.
In many applications it is important to assure that the entire surface is coated. The amount of material that is applied to a surface is usually measured in terms of coating weight. In an ongoing effort to reduce costs, coating weights have also been reduced. However, lower coating weights necessitate tighter control over the coating process. There are many process variables that affect coating weight, including temperature, pressure, viscosity, spray duration, nozzle flow rate and pattern control, and spray applicator position. In typical known rotating coating application systems, each deposition of material onto the circumferential surface of the container body is called a wrap. In a known can coating system, a can may be coated with a single wrap or two or more wraps including fractional or partial wraps.
The amount of material that is applied to a rotating surface is a function of the above noted process variables, the number of wraps, and also the rotation speed of the surface. If the rotation speed were always a known constant, then the amount of material applied to the surface could be better controlled within the ability of the manufacturer to control the other process variables. As such, the other process variables noted above have a significant impact on the coating weight and completeness of each wrap. For example, the actual spray duration can have a major impact on the amount of coating material applied to the rotating surface as a function of the speed of rotation. Spray duration refers to the time duration that coating material impinges the surface being sprayed. Spray duration is thus affected by flow characteristics of material through the spray application device, material transport times and spray device turn on and turn off time delays. The turn on time delay refers to the time delay between the command to turn the spray application device on via a first trigger signal to the spray application device and the actual time that material begins to impinge the surface. Turn off delay refers to the time delay between the command to turn the spray application device off via a second trigger signal to the spray application device and the actual time that material stops impinging on the surface. If the rotation speed is not constant, the spray duration time greatly impacts the completeness of the wraps and the distribution of coating weight applied during each wrap.
In can coating operations, it is common to support a can on a spray machine when coating the interior of the can. The spray machine supports a number of cans and sequentially indexes them past one or two spray guns that coat the cans. The can is normally supported on a mandrel by the force of a vacuum. Thus the mandrel is referred to as a vacuum chuck. The vacuum chuck rotates the can at a desired speed during the time that the can is stopped in front of the spray gun. Normally, the can is completely coated after being rotated two or three revolutions while being sprayed with coating material. Each complete revolution is called a “wrap”.
An existing system for monitoring and controlling the coating of the can on a spray machine is the Nordson iTrax® System. This system is available for purchase from Nordson Corporation, Amherst, Ohio and is described, at least in part, in International Publication Number WO 2005/016552 A2, published Feb. 24, 2005 that is hereby incorporated by reference in its entirety.
Even though a can is set to rotate at a desired speed on a spray machine as described above, the can may not be spinning properly. If the can is not rotated properly during the spraying operation, it may not be properly coated. It is also possible that even though the can is properly rotated, it is not properly sprayed. Improperly coated cans must be detected before they are filled with a food or beverage and sold to a consumer.